Method of making nodular iron castings

ABSTRACT

The method of producing a nodular cast iron casting comprising the melting of a cast iron bath of low sulphur content and near eutectic composition with a chill value of no more than eight thirty-seconds inch as measured by a standard wedge test. The bath is preconditioned by adding a rare earth and an alkaline earth containing alloy in an amount sufficient to increase the chill value from 50 to 150 percent and then pouring the bath into a mold having at least one reservoir and a dam skimmer gate with the reservoir containing from one-fourth to 1 1/4 percent by weight of a magnesium alloy. The last mentioned alloy is sufficient in amount to retain at least 0.01 percent magnesium in the metal from the bath which passes through the alloy so as to produce a casting containing nodular graphite.

United States Patent 1191 Moore 1 Oct. 16, 1973 [76] Inventor: William H. Moore, Winker Ln.,

Westport, Conn.

22 Filed: Nov. 1, 1972 21 Appl. No.: 302,762

3,001,869 9/1961 Longstreth... 75/129 2,144,200 l/1939 Rohn 75/57 2,841,488 7/1958 Morrogh 75/130 A 2,792,300 5/1957 Livingston 75/130 A 2,542,655 2/1951 Gagnebin..... 75/130 A 2,980,530 4/1961 Crome 75/57 3,703,922 11/1972 Dunks 75/130 R 3,498,361 3/1970 Hall 75/130 R OTHER PUBLICATIONS Schaum, Ductile Iron 25 Year Saga of Success,

Modern Casting (May, 1973), Pp. Dl-l DI-32.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg AttorneyGeorge V.- Woodling et a1.

57] ABSTRACT The method of producing a nodular cast iron casting comprising the melting of a cast iron bath of low sulphur content and near eutectic composition with a chill value of no more than eight thirty-seconds inch as measured by a standard wedge test. The bath is preconditioned by adding a rare earth and an alkaline earth containing alloy in an amount sufficient to increase the chill value from 50 to 150 percent and then pouring the bath into a mold having at least one reservoir and a dam skimmer gate with the reservoir containing from one-fourth to 1 4 percent by weight of a magnesium alloy. The last mentioned alloy is sufficient in amount to retain at least 0.01 percent magnesium in the metal from the bath which passes through the alloy so as to produce a casting containing nodular graphite.

6 Claims, 6 Drawing Figures PATENTED UN 16 I973 SHEEI 20F'3 FIG.2A

FIG. 2c

FIG. 2

METHOD OF MAKING NODULAR IRON CASTINGS My invention relates to nodular iron castings and more particularly to an improved method of producing nodular graphitecast iron of high as-cast ductility and superior mechanical properties.

It has for its object a method of incorporating nodularizing alloys into metal without pyrotechnics.

Another object is to avoid the effects of structure deterioration which is common with conventionally made nodular irons.

Another object is to produce a nodular iron casting free from carbides in the as-cast condition and having a high nodule" count and a high as-cast ductility.

Another object is to provide a method where the extent of nodularization can be effectively adjusted to suit the casting section being produced and to avoid the occurrence of mixed and inferior graphite structures in the casting.

Other objects in this invention will become apparent from the specification and drawings in which:

FIG. 1 is a drawing showing the gating system for three test bar castings, A, B and C. 1, 2 and 3 are the three test bars; 4, 5 and 6 are ingates into these bars from the runner bar 7 which is connected to the dam gate 8 having the alloy 9 in its reservoir and connected to a downsprue 10. These test bars were cast according to the method of this invention.

FIGS. 2A, 2B and 2C are a set of photomicrographs of the structures obtained in bars A, B, and C shown in FIG. 1 and cast without the benefit of Step No.2 of this invention. These are marked 2A, 2B and 2C. 2A is the structure obtained in the Bar A at a point farthest from the ingate 4. Bars B and C showed similar structures at the same points farthest away from ingates 5 and 6. 2B is the structure obtained in the center of Bar A in the area where the tensile bar was machined. 2C is the structure obtained in Bar B in the center where the tensile bar was machined. This is essentially the same structure as found in Bar C in the center.

FIGS. 3A and. 38V are a set of photomicrographs taken from the bars of FIG. 1, but where Step No. 2 of the method of this invention was used. 3A represents the structure taken from the center of Bar A, while 38 is the structure taken from the center of Bar C. The structure from the center of Bar B was essentially the same as that shown in 3A and 3B.

The conventional method of making nodular iron is to add any one or any combination of various wellknown nodularizing agents particularly cerium, magnesium and calcium alloys to molten metal in sufficient quantity to reduce the sulfur content to a low value and leave residual nodularizing elements in the composition to promote the formation of graphite or carbon in the nodular form.

Most of the methods used in commercial practice today involve some form of desulphurization of the molten metal before nodularization to reduce costs, improve consistency and provide a nodular iron which is cleaner and exhibits less shrinkage and tendency for defects such as carbon flotation and dross.

Unfortunately, nodular iron contains volative nodularizing agents such as magnesium which gradually volatilizes from the metal thereby decreasing the degree of nodularization. Additionally, the graphitizing effect of silicon alloys added which is part of most procedures for making nodular iron also tends to fade with time, leading to a product containing free carbides which are nonmachinable and brittle.

Because of this, it is necessary to add excess agent such as magnesium to allow for the loss and it is necessary also to cast the nodular iron in a relatively short period of time to prevent deterioration of the degree of graphitization or nodularization. Excess agent promotes various foundry defects such as dross and high solidification shrinkage. It also increases the cost of the nodular iron.

The addition of nodularizing alloys particularly those containing magnesium is accompanied by violent pyrotechnical action which is harmful from a standpoint of ecology and which because of its violence, is difficult to control quantitatively.

Various methods have been proposed to overcome these basic objections to current procedures for making nodular iron and some of these methods are successful to a limited degree in some respects.

One of the methods now being used is that of incorporating the graphitizing silicon alloy or the nodularizing alloy or both directly into the mold gating system. In this way, pyrotechnics are avoided and decay of structure is avoided because additions are being made at the last possible moment, that is, during the casting operation.

Unfortunately, however, the addition of nodularizing alloy particularly those containing magnesium in the mold has several drawbacks in terms of consistency of result. This seriously limits extensive use of this procedure.

The methodof my invention also proposes addition of at least part of the nodularizing alloy directly into the gating system of the mold but is an integrated process comprising a sequence of events and steps that will result in a consistent end product avoiding the difficulties presently associated with nodularizing in the mold. My process is best described and illustrated by some tests conducted to illustrate and highlight the difficulties associated with methods now being used by those skilled in the art.

In a special test three test bar castings were molded in a single mold and were connected with a gating system as shown in FIG. 1. Magnesium ferrosilicon sized to one-fourth to one-eighth inch mesh as preferred by those skilled in the 'art and equal to three-fourth percent by weight of the metal poured was placed in a suitable pocket or reservoir 9 fitted with a dam skimmer core to avoid products of reaction entering the test bars, and a nodular base iron of sulphur content of under 0.01 percent was poured into this mold. The solidified test bars were then machined and examined for physical properties and structure. These bars were identified by the letters A, B and C according to the position they had occupied in the mold.

Table I shows results obtained and FIGS. 2A, 2B and 2C show the structures involved in these test bars.

TABLE 1 Bar A Bar B Bar C Tensile Strength (psi) 6l,l88 47,202 44,205 Elongation 2L9 7.8 6.2 Silicon Content 2.42 2.03 L74 Magnesium Content '70 0.048 0.046 0.040

Bar A was satisfactory in every way. Bar C was unsatisfactory, although part of it had a suitable chemical analysis and Bar B was unsatisfactory although the structure in one area was every bit as good as that in Bar A.

It is evident from this test that a casting made by nodularizing in the mold may be quite unsuitable in some areas and that a homogeneously nodular structure does not always result. It follows also that this problem of nonuniformity may be present whenever a complex casting is made by this method or where several castings are made from the same gating system, which is common practice in anycommercial foundry.

In a second test the same gating system was used as in the first test, but in this case, the metal was run into a test ladle and not into the mold cavity. This was done so a sample could be collected and poured at different times during the pouring cycle. The total pour of equal weight to that of the first test took seconds. Samples were collected and analyzed giving the results shown in Table 2.

TABLE 2 TIME SILICON MAGNESIUM seconds This test clearly illustrates that absorption of magnesium alloy by the metal flowing through a gating system is not uniform and that extreme differences can result in various areas of single castings or in different castings from the same gating system when made this way.

Several other tests of a similar nature were run in which the size of the alloy, the rate of pouring and the dimensions of the alloy chamber were varied in an effort to get equal treatment of all metal running through the system'; while some improvement did result, it was found that it was virtually impossible to obtain uniformly treated metal through the gating system over the time period taken to pour a casting. This means that conventional procedures for nodularizing in the mold suffer from the disadvantage of nonuniform treatment, which can seriously affect the quality of the castings made.

My invention is based on the discovery that magnesium cannot be added in a completely uniform manner in the gating system of a mold, and that the ill effects of nonuniform magnesium content can be negated by careful pre-treatment of the molten metal in the ladle prior to casting it in a mold containng magnesium alloy. This can be done in a manner that avoids pyrotechnics and fading decay during the normal useful pouring life of the metal in the mold and in a manner that insures uniformly good results in the casting.

There are three basic steps in making a nodular casting by my method:

1. Melting the charge and desulphurizing the metal.

2. Adding a selected nodularizer of low pyrotechnic value and high fade resistance to the metal in the ladle.

3. Pouring the casting in a mold having a gating system containing a magnesium silicon alloy.

Each of these steps is a necessary part of the invention, but each by itself is meaningless. It is only when each is conducted properly and combined in the process that the product. of my invention results.

The first step of the method of my invention involves melting a base iron of conventional chemistry for nodular iron and of low sulphur content. This involves a carbon content ranging from 3.30 to 4.00, a silicon content from 1.0 to 3.0, a manganese content of 0.20 to 1.00, a phosphorous content of 0.02 to 0.10 and a sulphur content of 0.005 to 0.06 percent. Chemical contents outside of these limits may also be used for special purposes. Alloys for special effects such as nickel, molybdenum or copper may also be used. The important feature is that the iron with. respect to carbon and sili con should be at or near eutectic in composition, having a carbon equivalent ranging from about 4.0 to 4.6 percent. This is considered as conventional practice for nodular iron. I prefer a chill value of less than eight thirty-seconds inch as measured on a standard wedge test. Melting may be conducted in any furnace, that is, electric furnace, cupola or reverberatory furnace which may be acid or basic lined according to preference. Sulphur content is preferably low in the interests of economy and metal cleanliness, but the initial sulphur content allowable depends on whether or nor desulphurization by external means such as the use of soda ash or calcium carbide is contemplated.

While Step 2, where a nodulariz ation agent is added, will reduce the sulphur content to a value well below 0.02 percent, it is normal to first reduce this sulphur by other less expensive means. I prefer a sulphur content ofless than 0.025 percent and even less than 0.015 percent before Step 2 of this method is conducted.

The method I prefer to reduce sulphur to this value is the use of calcium carbide as a clesulphurizer and a porous plug as a means of agitation of the metal. This procedure is well-known to those skilled in the art.

The second step of the process of my invention is the critical one in that I have found that when successfully conducted, it pre-conditions the metal so that possible ineffective nodularization in the gating system of the mold is unlikely to occur. I have, in fact,-conditioned the metal in the ladle so that the process of nodularizetion in the mold is less critical and the end product is a more uniform nodular casting.

The second step of the invention consists of adding an agent containing rare earth elements mainly cerium and alkaline earth elements, mainly calcium, to the metal in the ladle so as to change the chill value of the metal to a value related to the initial chill value of the bath. What I wish to do is increase this chill value by a definite amount.

There are two reasons for this. The first is that calcium and cerium will not increase the chill value until they have first effectively neutralized the sulphur content. When the sulphur content has been neutralized, calcium and cerium begin to produce metastable carbides in the melt thereby increasing the chill value. As the chill value may be quickly and easily measured, a very effective control tool in pre-conditioning the metal for subsequent nodularization in the mold becomes available. This is an essential feature of the process of my invention.

I have found that the most convenient method of introducing calcium and cerium into the melt is to use a mixture of calcium silicide and cerium fluoride or a mixture of calcium carbide and cerium fluoride. In some cases I may also add a small quantity of magnesium in this step, but it is not an essential part of the process as is cerium, which gives a very definite increase in chill value when it is present in sufficient amount.

As already mentioned, the original chill value of the bath as measured on a standard test wedge such asare with a one-half inch back and a 28 wedge angle is preferred at no more than eight thirty-seconds. This is because higher chill values may lead to the production of stable carbides rather than metastable carbides when Step 2 is conducted by adding cerium and calcium. Stable carbides would adversely affect the structure in the final castings. The only way to be sure that metastable carbides are present is to start with a metal of high graphitizing value that is a low chill value and then cause this chill value to increase because of the cerium and calcium added. l actually prefer a base metal chill value of one thirty-second to four thirty-seconds inch although a value as high as eight thirty-seconds inch would still be acceptable.

Calcium and cerium are added to the bath in Step 2 until the chill value increases by at least 50 percent but preferably by not more than 150 percent. Expressed in terms of actual chill values, this means the chill is increased from the range of one thirty-second to eight thirty-seconds to a value falling between 1.5/32 and twenty-four thirty-seconds. The important feature is that the chill value is increased over that that existed before the addition of calcium and cerium was made.

Thus, if the initial chill value was two thirty-seconds, the increased value would be three thirty-seconds to five thirtyseconds. Actually, I strive to increase the chill value by about 100 percent so as to avoid errors caused by chill value measurement, giving a false picture of Step 2 of this invention. Thus, I normally try to add sufficient calcium and cerium to double the chill value over what it was before the addition. Thus, ideally a bath of initial chill value of four thirty-seconds inch would be changed to a chill value of eight thirtyscconds inch of higher by a successful Step 2 treatment ofthis invention.

A typical mixture used for Step 2 of this invention would consist of 80 percent calcium silicide and percent rare earth (cerium) fluoride. An amount that would increase the chill value as desired would range from one-eighth to 1 1/8 percent and more usually about three-fourths percent depending on the sulphur content of the bath. Other combinations of calcium and cerium are also effective, but this is a matter of local preference and economics. The important feature of Step 2 of this invention is to introduce sufficient calcium and cerium into the melt to give a measurable increase in the chill value of the metal. When this has been done, the metal has been suitably pre-conditioned for final Step 3, which consists of complete nodularization in the gating system of the mold. Another rare earth which may be used is neodymium and another alkaline earth material is barium.

Step 3 of this invention consists of casting the metal of Step 2 in a mold which contains a magnesium-silicon alloy in a suitably designed gating system. The metal flows over the alloy and dissolves it on its way to that part of the mold constituting the casting itself. The dissolved alloy results in the production ofa fully nodular structure characterized by a large number of small perfectly formed nodules in a largely ferritic matrix and with the complete absence of hard brittle carbides. I have already disclosed that solution of this magnesium alloy may be variable over the complete pour of the casting, but my invention is based on the fact that preconditioning of the metal as in Step 1 and particularly Step 2 of my method reduces the relative importance of this nonuniform alloy solution to the point where a very uniform casting in terms of mechanical properties and structure results. Without such pre-conditioning, Step 3 could easily result in a casting so nonuniform as to be commercially unacceptable.

In Step 3 I prefer to use a dam gate arrangement well known to those skilled in the art, although other skimmer devices can also be used. This dam gate consists of a metal reservoir and a skimmer core which keeps all dirt and products of reaction from entering the mold cavity. The relative size of the reservoir is important in terms of the volume occupied by the magnesiumsilicon alloy. I prefer to size this reservoir so that the weight or volume of alloy used will occupy no less than 25 percent but no more than 50 percent of the volume of the reservoir. This ensures the best condition for uniform solution of this alloy during the pouring time of the casting. The rate of flow of metal is governed by the choke under the skimmer core and is according to normally acceptable foundry practice where the pouring rate is equal approximately to one to one and one/- fourth times the square root of the weight of metal poured. The alloy used may be any alloy containing magnesium and silicon and its preferred size is about one-fourth to one-eighth mesh because this size will dissolve reasonably uniformly. The magnesium-silicon alloy may be replaced with a magnesium-nickel alloy.

The amount of alloy added is dependent primarily on the duration of the pouring time of the mold but will generally be so that total retained magnesium in the final metal casting is at least 0.01 percent based upon the weight of metal and usually is not more than 0.10 percent. This means that more than this be available initially and I prefer that the magnesium initially available be from 0.04 percent to 0.20 percent based upon the weight of metal. The total amount of silicon or nickel available initially should be from 0.10 percent to 1.0 percent. l have found that these amounts of magnesium and silicon while not too critical, give the best results under most conditions.

I may use more than one reservoir dam gate in the gating system where the metal is poured relatively slowly and where the casting is complex and is gated at several different points. This selection of the exact gating system would relate basically to casting geometry as is well known to those skilled in the art. In the process of my invention, a multiple of dam gates may tend to give more uniform solution of the alloy which is always desirable but which has been rendered less critical by the pre-conditioning of the metal accomplished in Step 2 of the method of this invention. Without such preconditioning the solution rate and uniformity would become so critical that inferior castings could result. In the essential Step 2 of this process the criticality of Step 3 has been reduced to the point where it is possible to make commercially acceptable castings.

As an example of the workings of the method of this invention, 1 cast a similar series of test bars as described in FIG. 1 of this specification. The mold consisted of three test bars as in FIG. 1. The metal was melted and by test found to have the following characteristics:

TOTAL CARBON 3.80 SILICON 1.92% MANGANESE 0.35% PHOSPHOROUS 0.03% SULPHUR 0.03%

This metal was desulphurized using 1 percent calcium carbide and agitation supplied with a porous plug and nitrogen gas. The sulphur content was reduced to 0.01 percent and the chill value of the bath was measured and found to be three thirty-seconds inch on a wedge having a one-half inch back and a 28 angle.

The bath was then treated with an addition of fiveeighths percent of a mixture containing 80 percent 'calcium silicide and 20 percent rare earth fluorides. The sulphur content was further reduced to 0.005 percent and the chill value was increased to seven thirtyseconds inches. This pre-conditioned metal was cast into the mold where one-half percent by weight of oneeighth inch mesh, percent magnesium, 40 percent silicon alloy had been placed in the dam gate reservoir. The resultant test bar castings were numbered A, B and C as was the case in the experiment described in FIG. 1, and they were then tested to give the following results in Table 3.

TABLE 3 Bar No. Tensile Elonsilimagnegation con sium A 75,192 21.50 2.52 0.032 B 76,423 21.90 2.44 0.023 C 74,925 22.60 2.36 0.019

There was evidence of nonuniform solution of magnesium and silicon as shown by lower silicon and magnesium contents in Bar C as compared to Bar A. The tensile and elongation properties in the bars, however, were all of the high order of magnitude and all acceptable on a commercial basis. In the experiment previously described in FIG. 1 done by the conventional procedures known to those skilled in the art, the results had not been commercially acceptable. In the method of our invention where the metal was pre-conditioned as in Step 2, the nonuniformity resulting from nodularization in the mold was of no consequence in that uniform commercially acceptable mechanical properties resulted. The structures in Bars A, B and C consisted in each case of small well-formed nodules of graphite in a fully ferritic matrix. These structures are shown in FIGS. 3A and 3B of the specification.

By utilizing all the steps of the method of this invention, I am able to avoid fading or deterioration of the nodular structure of the castings. I am able to avoid the pyrotechnics of magnesium additions to the metal in the ladle and I am able to produce castings of uniformly commercially acceptable properties in the castings. The pre-conditioning of the metal in Step 2 of this invention by adding calcium and cerium to produce an increased chill value'is an essential prelude to magnesium addition in the gating system of the mold. With this preconditioning I am able to overcome the harmful effects that may readily result from nodularizing in the mold with magnesium alloy without such prior treat- 'm'ent.

I have made many castings by the method of this invention and have found the results to be exceptionally uniform. While structures generally tend to be ferritic in nature, I have also been able to produce fully pearlitic structures by including elements like tin or copper in thebase metal composition. Generally, however, I prefer to use my method where high as-cast elongations are desired. 7

I have described this invention in its preferred form with a good deal of particularity, but it is understood that modifications and variations apparent to those skilled in the art are considered to be within the preview and scope of the invention and appended claims.

What is claimed is:

l. The method of producing nondular cast iron castings comprising melting a cast iron bath of low sulphur content and near eutectic composition with a chill value of no more than eight thirty-seconds inch as measured by a standard wedge test, pre-conditioning said bath by adding a rare earth and an alkaline earth containing alloy in an amount sufficient to increase the chill value from 50 to percent and then pouring said bathinto a mold having at least one reservoir and dam skimmer gate with said reservoir containing from one-fourth to l 7 percent by weight based upon the weight of cast iron of a magnesium alloy, said last mentioned alloy being sufficient in amount to retain at least 0.01 percent magnesium based upon the weight of cast iron in the metal from said bath passing through said alloy so as to produce a casting containing nodular graphite.

2. The method of claim 1, wherein said cast iron bath has a carbon content from 3.30 to 4.00 percent, a silicon-content from 1.0 to 3.0 percent, a manganese content from 0.20 to 1.00 percent, and a phosphorous content from 0.02 to 0.10 percent, said cast iron in said bath or after said pre-conditioning step having a sulphur content from 0.005 to 0.06 percent.

3. The method of claim 2, wherein said rare earth comprises cerium or neodymium and said alkaline earth comprises calcium or barium.

4. The method of claim 3, wherein said cerium is cerium fluoride and said calcium is calcium silicide.

5. The method of claim 3, wherein said magnesium alloy is magnesium silicon alloy or magnesium nickel alloy.

6. The method of claim 4, wherein the volume of said alloy in said reservoir will occupy no less than 25 percent and no more than 50 percent of the volume of said reservoir. 

2. The method of claim 1, wherein said cast iron bath has a carbon content from 3.30 to 4.00 percent, a silicon content from 1.0 to 3.0 percent, a manganese content from 0.20 to 1.00 percent, and a phosphorous content from 0.02 to 0.10 percent, said cast iron in said bath or after said pre-conditioning step having a sulphur content from 0.005 to 0.06 percent.
 3. The method of claim 2, wherein said rare earth comprises cerium or neodymium and said alkaline earth comprises calcium or barium.
 4. The method of claim 3, wherein said cerium is cerium fluoride and said calcium is calcium silicide.
 5. The method of claim 3, wherein said magnesium alloy is magnesium silicon alloy or magnesium nickel alloy.
 6. The method of claim 4, wherein the volume of said alloy in said reservoir will occupy no less than 25 percent and no more than 50 perceNt of the volume of said reservoir. 